The speaker addresses a little bird and asks it, gently, to stop singing. The song doesn't feel light or cheerful to the speaker. It drops "like pebbles down the well of the heart," an image that suggests each note sinks deep into a place that is already hollow and sore, and that the listener can almost hear the echo.
The middle lines widen the view. The nightingale and the swan are both famous singers, and the speaker says that sorrow and death inspire their art. That idea is a quiet comfort and a quiet warning: grief can produce beautiful song. Yet the poem ends by saying this doesn't last, repeating "But not for long" twice. The repetition sounds like a sigh, or like someone trying to reassure themselves.
The mood is hushed, wistful and tender. The speaker seems weighed down by sadness, and the poem suggests that even beautiful songs born from pain fade, and that sorrow itself may pass too.